Organic electroluminescent devices have superior characteristics, such as low driving voltage (e.g., 10V or less), broad viewing angle, rapid response time and high contrast, over liquid crystal displays (LCDs), plasma display panels (PDPs) and inorganic electroluminescent display devices. Based on these advantages, organic electroluminescent devices can be used as pixels of graphic displays, television image displays and surface light sources. In addition, organic electroluminescent devices can be fabricated on transparent flexible substrates, which can reduce the thickness and weight thereof and have good color representation. Therefore, in recent years, organic electroluminescent devices have gradually been used in flat panel displays (FPDs).
A representative organic electroluminescent device was reported by Gurnee in 1969 (U.S. Pat. Nos. 3,172,862 and 3,173,050). However, this organic electroluminescent device suffers from limitations in its applications because of its limited performance. Since Eastman Kodak Co. reported multilayer organic electroluminescent devices capable of overcoming the problems of prior-art devices in 1987, remarkable progress has been made in the development of the organic electroluminescent technique.
Such organic electroluminescent devices comprise a first electrode as a hole injection electrode (anode), a second electrode as an electron injection electrode (cathode), and an organic light-emitting layer disposed between the cathode and the anode, wherein holes injected from the anode and electrons injected from the cathode combine with each other in the organic light-emitting layer to form electron-hole pairs (excitons), and then the excitons fall from the excited state to the ground state and decay to emit light. At this time, the excitons may fall from the excited state to the ground state via the singlet excited state to emit light (i.e. fluorescence), or the excitons may fall from the excited state to the ground state via the triplet excited state to emit light (i.e. phosphorescence). In the case of fluorescence, the probability of the singlet excited state is 25% and thus the luminescence efficiency of the devices is limited. In contrast, phosphorescence can utilize both probabilities of the triplet excited state (75%) and the singlet excited state (25%), and thus the theoretical internal quantum efficiency may reach 100%. Therefore, it is crucial to develop highly efficient phosphorescent material, in order to increase the emissive efficiency of an organic electroluminescent device.
Currently, the main luminescent materials of the organic electroluminescent devices are small-molecule materials due to higher efficiency, brightness and life-span of the small-molecule organic electroluminescent devices than the polymer light-emitting diodes (PLEDs). A small-molecule organic electroluminescent device is mainly fabricated by way of vacuum evaporation rather than spin coating or inkjet printing like PLEDs. However, the equipment cost of the vacuum evaporation is high. Additionally, 95% of the organic electroluminescent materials are deposited on the chamber wall of the manufacturing equipment, such that only 5% of the organic electroluminescent materials are coated on a substrate, resulting in high manufacturing cost. Therefore, a wet process (such as spin coating or blade coating) has been provided to fabricate small-molecule organic electroluminescent devices to reduce equipment costs and improve the utilization rate of organic electroluminescent materials.
Therefore, for the organic electroluminescent technique, it is necessary to develop soluble organic phosphorescent materials which are suitable for use in a wet process.